1. Field of the Invention
The present invention relates to a zoom lens that is suitable as a photographic optical system of an image pickup apparatus, such as a digital still camera, a video camera, a TV camera, and a monitoring camera.
2. Description of the Related Art
As a photographing optical system used for an image pickup apparatus, there is required a zoom lens that has a short total lens length, a small size, a wide angle of view, a high zoom ratio, and a high resolution.
As a compact zoom lens having a wide angle of view and a high zoom ratio, a negative-lead type zoom lens is known. In the negative-lead type zoom lens, a lens unit having negative refractive power is located closest to an object side.
As a negative-lead type zoom lens, a three-unit zoom lens is known. The three-unit zoom lens includes, in order from an object side to an image side, a first lens unit having negative refractive power, a second lens unit having positive refractive power, and a third lens unit having positive refractive power. There is known a compact three-unit zoom lens having a wide angle of view, in which a first lens unit includes a negative lens and a positive lens, a second lens unit includes three lenses, and a third lens unit includes a single positive lens.
Japanese Patent Application Laid-Open No. 2011-064933 discusses a compact zoom lens in which a focal length equivalent to a silver-halide camera is 29 mm and a zoom ratio is about 4.7 times. U.S. Patent Application Publication No. 2010/0214665 discusses a compact zoom lens in which a focal length equivalent to a silver-halide camera is 26 mm and a zoom ratio is about 3.8 times.
In the negative-lead type three-unit zoom lens, it is effective to shorten a focal length of, for example, a second lens unit that manages a main variable magnification to downsize the entire zoom lens and achieve a high zoom ratio. However, if the focal length is simply shortened, Petzval sum is increased. Therefore, it is difficult to realize a characteristic of a flat image plane over the entire zoom range. It is effective for suppressing an increase in Petzval sum to share positive refractive power with a plurality of positive lens, or use a high-refractive-index material for a positive lens.
In each exemplary embodiment of Japanese Patent Application Laid-Open No. 2011-064933, an average refractive index of materials of positive lenses included in the entire zoom lens is about 1.71. In each exemplary embodiment of U.S. Patent Application Publication No. 2010/0214665, an average refractive index of materials of positive lenses included in the entire zoom lens is about 1.74. In these lens configurations, if a high zoom ratio and downsizing of the entire zoom lens are intended to be achieved by shortening a focal length of a second lens unit, curvature of field increases over the entire zoom range, and it is difficult to realize a characteristic of a flat image plane.
For reducing the curvature of field in the entire zoom range while achieving downsizing of the entire zoom lens and high zoom ratio, it is important to appropriately set a lens configuration of each lens unit constituting the zoom lens, or refractive power of each lens unit. For example, it is important to appropriately set refractive power of a second lens unit, or a material of a positive lens included in the entire zoom lens. If the setting is inappropriate, it is very difficult to obtain high optical performance while achieving downsizing of the entire zoom lens, a wide angle of view, and a high zoom ratio.